As is known, one of the advantages of cellular radio communication systems is that the system bandwidth is much greater than the bandwidth of any individual cell. Indeed, to a first approximation, the aggregate bandwidth of such a system can approach the sum of the bandwidths of the individual cells. This is an important feature of existing cellular systems because it enables them to service far more traffic than any single cell is required to handle, thereby reducing the cost and complexity of the individual cells and of the overall system.
Client access (i.e., user and networked device access) to known cellular systems typically is provided by far field coupled radio links. The far field electric and magnetic field strengths of a radiating element, such as an antenna, fall-off as the inverse first power of the propagation distance, r (i.e., 1/r), so the power falls-off as 1/r.sup.2. Conventionally, neighboring cells within these far field coupled cellular systems operate on different carrier frequencies (i.e., different "channels"), thereby ensuring that cells operating on like channels are spatially separated from each other by buffer zones of sufficient size to maintain an adequate signal-to-interference level throughout each of the cells. To accommodate this strategy, mobile clients are switched from channel-to-channel as they move from cell-to-cell, so sophisticated switching is required to perform these "hand-off" transitions gracefully. Clearly, therefore, significant cost savings could be realized if all cells operated on the same channel or channels. Unfortunately, however, the spatial rate at which the radiated power rolls-off in these far field systems is too gradual (see FIG. 1) to permit the use of identical channels in adjacent cells, without incurring markedly degraded performance near the boundaries of the cells and/or "dead" zones between them.
Another disadvantage of using far field coupled radio links in local communication systems, such as cellular LANs, is that these systems often are installed in environments containing various rf reflectors, such as in buildings having metal wall studs. For example, far-field coupled cellular systems generally operate at UHF frequencies to enable modest sized antennae to function as efficient radiators. However, UHF signals have wavelengths on the order of only one foot or so, which means that even relatively small objects can cause significant reflections. In buildings or similar sites, where many of the rf reflectors are spatially separated from the radiating source and from each other by distances that are relatively small multiples of the signal wavelength, such as the reflectors in and near the walls of a small room or office, such rf reflections tend to produce strong resonances. Consequently, the standing wave patterns that are associated with these resonances tend to materially perturb the radiated fields in accordance with a complex and difficult to predict pattern (see FIG. 2). Moreover the problems that are caused by these standing wave patterns would be compounded if adjacent, spatially contiguous cells were operated on the same carrier frequencies because the standing wave patterns that are generated within the adjacent cells then would cause the transitions from cell-to-cell to be non-monotonic.
Clearly, reliable wireless communications are needed to give portable computers and other mobile devices convenient access to LANs and similar services, without detracting .from the portability of such devices. It also is clear that it would be extremely beneficial to be able to take advantage of the aggregate bandwidth capabilities of cellular systems in LANs, so that the bandwidth demands of multiple users can be satisfied economically.